This invention relates to pneumatic tires, and more particularly to pneumatic tires having a high resistance to cut growth, which are used in vehicles running on a road, at least a part of which includes a rough road being apt to give cut failure to the tire. Especially, it relates to the tires of this type having an improved performance against cut damage in applications for construction truck, dump truck and the like.
Since the tires of this type are mainly used on rough roads not arranged as a running road, the tire body, particularly the side portion thereof is frequently subjected to cut damage due to the contact with sheer stones and rocks, pieces of broken metal, stubs after felling or the like. When such cut damage is once produced, this damage gradually grows with the continuous running of the tire and proceeds into the inside of the tire and finally arrives at the carcass layer, whereby separation failure of carcass ply or plies constituting the carcass layer is caused. Alternatively, the cut damage further penetrates over the carcass layer into the inside of the tire to cause air leakage.
In general, a risk of causing the above failures tends to be higher in the side portion of the tire than in the crown portion thereof because the side portion is quite in a defenseless state as compared with the crown portion which is provided with a thick tread rubber and reinforced with a belt or breaker layer. Further, this tendency is more conspicuous in the side portion of pneumatic radial tires comprising a carcass layer of a so-called radial structure and a belt layer superimposed thereabout, wherein the carcass layer is composed of at least one rubberized ply each ply containing steel cords arranged substantially parallel to the radial plane of the tire and the belt layer has a high circumferential rigidity.
As a countermeasure against cut damage as well as the separation failure of carcass ply and the air leakage failure accompanied therewith, there have hitherto been made various attempts, but satisfactory results have not been attained by anyone of these attempts.
For instance, there is a first-type attempt wherein a reinforcing cord layer of fiber material is disposed in the side portion of the tire outside the carcass layer to make the depth of side cut damage shallow as disclosed in U.S. Pat. No. 3,703,203. In this case, the effect for preventing cut damage itself is improved to some extent, but with the cut damage is once caused, the growth of such cut damage cannot be prevented and further the separation failure is apt to occur from the end of the reinforcing cord layer. The similar result fundamentally results even when the cord of the reinforcing cord layer is composed of metallic material.
As disclosed in U.S. Pat. No. 3,457,981, a second-type attempt is wherein a rubber swollen portion is disposed in a region extending from the shoulder portion of the tire to the side portion thereof to prevent the side cut. In this case, the protection for the side portion is effectively attained to some extent, but when the cut damage is caused once likewise the first-type attempt, the growth of such cut damage cannot be prevented.
As a third-layer attempt, a thick rubber layer is disposed in the side portion of the tire inside the carcass layer as disclosed in U.S. Pat. No. 3,464,477. In this case, it is anticipated that the rubber layer prolongs a time until air leakage is caused after the occurrence of cut damage, but the growth of the cut damage cannot basically be prevented. Therefore, the arrangement of the rubber layer does not sufficiently contribute to improve the resistance to air leakage. Furthermore, the increase of the thickness of the rubber layer increases the total amount of rubber to increase the heat generation. The strain applied to the bead portion and the end portion of the belt layer is increased and as the result, the durability of the tire is deteriorated and the production becomes difficult and the cost is increased.
In the light of the above mentioned drawbacks of the prior art, according to the invention, a tire mounted on vehicles running on rough road, particularly the side portion thereof is unavoidably subjected to cut damage to some extent. So, an object of the present invention is to obviate various drawbacks in the above described prior art and to highly improve the performance for preventing the air leakage of the tire and further the separation failure of the carcass ply without sacrificing other tire performance by advantageously preventing the growth of the cut damage which is subjected to the tire, particularly the side portion of the tire mounted on vehicles running on rough roads and penetrates toward the inside of the tire.
As a result of many experiments and various investigations, the inventors have found that if a fault surface having an extremely low adhesion or a middle rubber layer having a modulus lower than that of the adjoining rubber is provided at the interior of the rubber layer of the tire, the crack growth from the outer surface of the rubber layer is blocked by the fault surface of the low modulus middle rubber layer.